DNVGL-CG-0127 Finite element analysis

1 The electronic pdf version of this document, available free of chargefrom , is the officially binding GL ASCLASS GUIDELINEDNVGL-CG-0127 Edition October 2015 Amended February 2016 Finite element analysisFOREWORDDNV GL class guidelines contain methods, technical requirements, principles and acceptancecriteria related to classed objects as referred to from the rules. dnv gl AS October 2015 Any comments may be sent by e-mail to any person suffers loss or damage which is proved to have been caused by any negligent act or omission of dnv gl , then dnv gl shallpay compensation to such person for his proved direct loss or damage. However, the compensation shall not exceed an amount equal to tentimes the fee charged for the service in question, provided that the maximum compensation shall never exceed USD 2 this provision " dnv gl " shall mean dnv gl AS, its direct and indirect owners as well as all its affiliates, subsidiaries, directors, officers,employees, agents and any other acting on behalf of DNV - currentClass guideline DNVGL-CG-0127 .

2 Edition October 2015, amended February 2016 Page 3 Finite element analysisDNV GL ASCHANGES CURRENTThis is a new February 2016 General O nly editorial corrections have been correctionsIn addition to the above stated changes, editorial corrections may have been guideline DNVGL-CG-0127 . Edition October 2015, amended February 2016 Page 4 Finite element analysisDNV GL ASCONTENTSC hanges 3 Section 1 Finite element 51 2 Global strength Objective and 102 Global structural FE 103 Load application for global FE analysis 3 Partial ship structural Objective and 222 Structural Boundary 374 FE load combinations and load 405 Internal and external Hull girder analysis 4 Local structure strength 691 Objective and 692 Structural 693 Loads and boundary 755 analysis 5 Beam 771 Model 1 Class guideline DNVGL-CG-0127 .

3 Edition October 2015, amended February 2016 Page 5 Finite element analysisDNV GL ASSECTION 1 Finite element ANALYSIS1 GeneralThis class guideline describes the scope and methods required for structural analysis of ships and thebackground for how such analyses should be carried out. The class guidelines application is based on relevantRules for Classification of dnv gl Rules for Classification of Ships may require direct structural strength analyses as given in analyses carried out in accordance with the procedure outlined in this class guideline will normallybe accepted as basis for plan the text refers to the Rules for Classification of Ships, the references refer to the latest edition of theRules for Classification of case of ambiguity between the rules and the class guideline, the rules shall be recognised Finite element software may be utilised provided that all specifications on mesh size, elementtype, boundary conditions etc.

4 Can be achieved with this computer wave loads are calculated from a hydrodynamic analysis , it is required to use recognised software. Asrecognised software is considered all wave load programs that can show results to the satisfaction of DNV Objective of class guidelineThe objective of this class guideline is: To give a guidance for Finite element analyses and assessment of ship hull structures in accordance withthe Rules for Classification of Ships. To give a general description of relevant Finite element analyses. To achieve a reliable design by adopting rational analysis Calculation methodsThe class guideline provides descriptions for three levels of Finite element analyses:a) Global direct strength analysis to assess the overall hull girder response, given in ) Partial ship structural analysis to assess the strength of hull girder structural members, primarysupporting structural members and bulkheads, given in ) Local structure analysis to assess detailed stress levels in local structural details, given in class guideline, dnvgl CG 0129, Fatigue assessment of ship structures, describes methods of local finiteelement analyses for fatigue provides descriptions for a 2 and 3 dimension beam analyses of ship Material propertiesStandard material properties are given in Table 1 Class guideline DNVGL-CG-0127 .

5 Edition October 2015, amended February 2016 Page 6 Finite element analysisDNV GL ASTable 1 Material propertiesMaterialYoung s Modulus[kN/m2]Poisson ValueShear Modulus[kN/m2]Density[t/m3] minimum yield stress ReH has to be related to the material defined as indicated in the rules, RU Table 1. Consequently, it is recommended that every steel grade is represented by a separatematerial data set in the model, as the materials are defined in the structural Global coordinate systemThe following co-ordinate system is recommended; right hand co-ordinate system, with the x-axis positiveforward, y-axis positive to port and z-axis positive vertically from baseline to deck. The origin should belocated at the intersection between aft perpendicular (AP), baseline and centreline.

6 The co-ordinate system isillustrated in Figure should be noted that loads according to the rules, RU SHIP refer to a coordinate system with adifferent x-origin (located at aft end (AE) of the rule length L). This coordinate system is defined in the rules,RU SHIP [ ]. Figure 1 Global coordinate Corrosion DeductionFE models are to be based on the scantlings with the corrosion deductions according to the rules, RU Table 1, as follows 50% corrosion deduction for ships with class notation ESP; 0% corrosion deduction for other capacity assessment based on FE analysis is to be carried out with 100% corrosion 1 Class guideline DNVGL-CG-0127 . Edition October 2015, amended February 2016 Page 7 Finite element analysisDNV GL Finite element typesAll calculation methods described in this class guideline are based on linear Finite element analysis of threedimensional structural models.

7 The general types of Finite elements to be used in the Finite element analysisare given in Table 2 Types of Finite elementType of Finite elementDescriptionRod (or truss) elementLine element with axial stiffness only and constant cross sectional area along thelength of the elementLine element with axial, torsional and bi-directional shear and bending stiffnessand with constant properties along the length of the (or plate) elementSurface element with in-plane stiffness and out-of-plane bending stiffness withconstant (or plane-stress) elementSurface element with bi-axial and in-plane plate element stiffness with constantthickness2 node line elements and 4/3 node plate/shell elements are considered sufficient for the representation ofthe hull structure.

8 The mesh descriptions given in this class guideline are based on the assumption that theseelements are used in the Finite element models. However, higher order elements may also be elements with inner angles below 45 deg or above 135 deg between edges should be with high aspect ratio as well as distorted elements should be avoided. Where possible, the aspectratio of plate/shell elements is to be kept close to 1, but should not exceed 3 for 4 node elements and 5 for 8node use of triangular shell elements is to be kept to a minimum. Where possible, the aspect ratio of shellelements in areas where there are likely to be high stresses or a high stress gradient, is to be kept close to 1and the use of triangular elements is to be case of linear elements (4/3 node elements) it is necessary that the plane stress or shell/plate element 'sshape functions include incompatible modes which offer improved bending behaviour of the modelledmember, as illustrated in Figure 2.

9 This type of element is required particularly for the modelling of webplates in order to calculate the bending stress distribution correctly with a single element over the full webheight. For global FE-models, the mesh description given in this class guideline is based on the assumptionthat elements with incompatible modes are used. Figure 2 Improved bending of web modelled with one element over heightFor the global, partial ship and fine mesh strength analyses, the assessment against stress acceptancecriteria is normally based on membrane (or in-plane) stresses of shell/plate elements. For the fatigueassessment, the calculation of dynamic stress range for the determination of fatigue life is based on surfacestresses of shell/plate 1 Class guideline DNVGL-CG-0127 .

10 Edition October 2015, amended February 2016 Page 8 Finite element analysisDNV GL Singularities in membrane elementsFor global FE analysis translatory singularities in membrane elements structures can be avoided by arrangingso-called singularity trusses as indicated in Figure 3. To avoid any load transfer by these trusses, loadapplication on the singularity nodes in the weak direction is to be suppressed. Some FE programs suppressthese singularities internally. Figure 3 Singularity Model checkThe FE model shall be checked systematically for the following possible errors: fixed nodes nodes without stiffness intermediate nodes on element edges not connected to the element trusses or beams crossing shells double elements extreme element shapes ( element edge aspect ratio and warped elements) incorrect boundary , verification of the correct material and geometric description of all elements is required.